Optical couplers are key components in optical networks. Optical couplers are used for routing signals from one waveguide to another and/or for splitting optical signals into two independent signals at a predetermined power ratio to be transmitted over two different waveguides. The most common and widely used form of optical coupler is the “evanescent coupler.” Advantages of such couplers include small size and low insertion loss.
FIG. 1 is a plan view of a prior art optical coupler 10. In a planar optical circuit, a conventional evanescent optical coupler 10 is formed on a substrate 12 by laying out two waveguides 14 and 16 having respective input ends 18 and 20 and respective output ends 22 and 24. Waveguides 14 and 16 include respective core regions 34 and 36 (“cores”) surrounded by a cladding region 40 (“cladding”). Coupler 10 includes a straight section 46 wherein waveguides 14 and 16 are parallel and separated by a small distance ΔX, which typically is on the order of a few microns. Coupler 10 includes input and output sections 50 and 52 as well as bend sections 60 and 62 formed in waveguides 14 and 16 to provide separations S1 and S2 between the waveguides at the input and output sections. Separations S1 and S2 and bend sections 60 and 62 are such that optical coupling only occurs between the waveguides in straight section 46.
In the operation of optical coupler 10, a lightwave 70 is inputted to and guided by one of the waveguides, say waveguide 14. The optical power (power) in lightwave 70 is not confined to within waveguide core 34 but extends into the surrounding cladding 40 with a power distribution that drops off exponentially with distance from the core. This power is referred to as the “evanescent tail” or “evanescent wave.” The maximum distance from core 30 where the evanescent tail can be practically sensed is referred to herein as the “coupling distance,” which is a function of the difference in the refractive indices between the core 30 and the cladding 40, as well as the transverse dimension of the core 30.
In straight section 46, waveguide 16 is within the coupling distance of waveguide 14, so that optical power is coupled from waveguide 14 to waveguide 16 via the evanescent tail of lightwave 70. The amount of power coupled from waveguide 14 to waveguide 16 is a periodic function of the distance along the propagation direction (i.e., the Z-direction). The amount of coupled power for a fixed distance along the Z-axis is a strong function of the “geometry” of the coupler 10, i.e., the distance ΔX separating waveguides 14 and 16 and the difference ΔN in the refractive indices between the core 30 and the cladding 40.
One of the most important applications of an optical coupler 10 involves splitting input lightwave 70 equally between waveguides 14 and 20 So that equal amounts of optical power are outputted from waveguides 14 and 16 at output section 52. This is known as “3 dB (decibel) coupling,” and such a coupler is referred to as a “3 dB coupler.” Unfortunately, it is very difficult to make a true 3 dB coupler because the degree of coupling is very sensitive to the coupler geometry. For example, a conventional 3 dB optical coupler requires a refractive index difference ΔN within four parts in ten-thousand of the design value to achieve a power split within 2% of true 3 dB coupling. A consequence of not achieving 3 dB coupling to within the design specification is that the residual power can produce cross-talk, which reduces the performance of the optical network.